The invention relates to a developing station of a processing system for printing plates, with at least one rubbing element and one counter-roller, as well as transport rollers arranged downstream of the rubbing element in the direction of passage of the printing plates.
Printing plate processing systems, in particular for negatively working printing plates, have in the developing stations rubbing elements which mechanically aid the detachment of the coating portions of the printing plate which have not been hardened by developing. The use of rubbing elements makes possible a substantial increase in the speed at which the printing plates pass through the processing system.
German Auslegeschrift No. 2,507,221 discloses a developing apparatus for printing plates which has a pair of run-in rollers for the application of a toner dispersion onto the charge image on the printing plate. Additionally, the developing apparatus has a distributor roller that evenly distributes the toner dispersion on the charge image. The distributor roller has a core of metal and a surface consisting of soft, porous, electrically non-conductive material. This soft surface is in contact with the printing plate during the printing plate passage through the developing zone. Furthermore, there is a pair of squeeze rollers for the removal of the unused toner dispersion, which is collected in a catch tray and returned to the pair of run-in rollers.
German Offenlegungsschrift No. 2,331,253 discloses a developing apparatus for electrophotographic copying materials which are passed under application rollers in a developing zone. The application rollers are wetted from above with a toner dispersion. At the end of the developing zone there is a pair of squeeze rollers which removes the excess toner dispersion from the surface of the copying material by squeezing.
The rubbing elements which are used in practice in all developing stations for printing plates are predominatly brush or plush rollers which consist of a metal core with a corresponding bristle or plush covering. Instead of brush rollers, rubbing elements in the form of beams which are moved to and fro in a transverse direction to the passing printing plate and which have a rubbing surface that is covered with plush and/or bristles may also be used. The drive of the rubbing elements is thus either performed rotationally, as in the case of brush rollers which can also execute a to and fro movement, or oscillatingly, as in the case of rubbing beams.
Owing to the relatively simple construction and simple adjustment, in practice rotating rubbing elements, i.e., brush rollers, are usually used. The brush rollers in this case run in the direction of plate passage thereby preventing a plate jam at the brush rollers which is a particular problem with printing plates of small thickness. This problem is unavoidable when the brush rollers rotate counter to the direction of plate passage.
Opposite the rubbing element is either a counter-roller or a deflector in the form of a table structure which functions as abutment for the printing plate. The gap between the rubbing element and the abutment can then generally be fixed by vertical adjustment of the rubbing element.
In the case of the known developing stations, the plane of passage of the printing plate runs horizontal from the gap between the brush roller and its abutment through the gap of the pair of transport rollers or the gaps of the pairs of transport rollers. The transport rollers are arranged at the end of the developing station and transport the developed printing plate into a further processing station.
In the case of both types of abutment for the brush roller, whether it is a counter-roller or a guide baffle in table form as the supporting area for the printing plate, a flipping up of the trailing plate edge occurs when the plate has passed the rubbing element. In particular with small plate thicknesses, this flipping up frequently results in deformation and thus unsuitability of the printing plate. This effect is promoted by the combined effects of high bristle stiffness of the brush covering, high brush roller rotational speed and increasing contact pressure of the rubbing element.
In practice, the attempt to eliminate this effect results in compromise solutions between the individual parameters, such as bristle stiffness and brush roller rotational speed, as well as contact pressure of the brush roller, or in the installation of special holddown devices which substantially minimize a flipping up of the trailing plate edge. For the complete avoidance of flipping up of the trailing plate edge, processing systems with reversible brush/plush rollers are being used ever more increasingly. However, the reversing operation of these rubbing elements has a serious deficiency, as such processing systems cannot be charged continuously with printing plates. To be able to introduce a further printing plate into such a processing system, in each case the previously fed printing plate must have passed the rubbing element or elements and the rotational direction of the rubbing elements have changed, as otherwise a plate jam would occur, at the first rubbing element. This results in considerable idle time in continuous processing of the printing plates and consequent reduction in the printing plate throughput of the processing system.